When I want to test that a subroutine is now in my namespace, I check that the subroutine isn't yet defined, do the import, then check that the subroutine is defined:

BEGIN {
ok( ! defined( &some_func, "some_func not defined yet" );
use_ok( 'MyModule::Common', ':all' );
ok( defined( &some_func, "some_func was imported" );
}
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or if I care about testing the import method because I'm doing something tricky:

BEGIN {
my $class =  'MyModule::Common';
ok( ! defined( &some_func, "some_func not defined yet" );
require_ok(  $class  );

ok( $class->import, "import returns true" );

ok( defined( &some_func, "some_func was imported" );
}
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If you're relying on %EXPORT_TAGS and what to make sure it exports everything it is supposed to (meaning, most likely, that you told it to export the right things, you can look in %EXPORT_TAGS and see if each of those things are now defined in your current namespace. That can be a bit messy though.

Generally, I like to give each function its own test file, where I do extensive testing on it. If it should be in the :all tag, I'll test that it is exported just like I did in my code example. I don't worry about testing the the :all tag in a single test. I let the individual subroutine tests figure that out.